ARC OF CONTACT 



69 



the tests the horsepower transmitted was maintained at a 

 constant figure, while the humidity was varied. Tests were 

 made at varying center distances. The tests showed that 

 as the humidity decreased the sum of the tensions in the belt 

 increased. The results of the tests, as taken from curves 

 presented in the paper are given in the following table : 



EFFECT OF HUMIDITY ON LEATHER BELTS 



It was shown that increasing the temperature as well as the 

 humidity tended to increase the length of the belt and so 

 to decrease the tension. The principal conclusions drawn 

 from the tests were as follows: i. If a belt be set up at a 

 moderate relative humidity, the tensions will not be exces- 

 sively lower at lower relative humidities, nor will there be 

 any great danger of the belt slipping at higher relative humid- 

 ities unless there are excessive temperature changes. 2. If 

 a belt be set up at any relative humidity with a spring or 

 gravity tightener, a load 50 per cent greater than the stand- 

 ard can be transmitted at either high or low humidity with- 

 out danger of slipping, stretching the belt or producing exces- 

 sive pressure on the bearings. 



To Find the Arc of Contact of the Belt on the Pulley. 

 The angle of contact of the belt on the smaller of a pair of 

 pulleys is the difference between 180 degrees and twice the 

 angle between the diameter perpendicular to the line joining 

 the centers of the pulleys and the radius drawn to the point 

 of tangency of the belt on the pulley. In Fig. 1 8 it is the 

 angle cbf= 180 2 (angle abc}. The angle abc is similar to 



